Microbes as Biofertilisers

What Biofertilisers Are and Why They Matter

Biofertilisers are living organisms -- bacteria, fungi or cyanobacteria -- that, when applied to seeds, plant roots or soil, increase the availability of essential plant nutrients, most importantly nitrogen. Plants need large amounts of nitrogen to build proteins, nucleic acids and chlorophyll, but although the atmosphere is nearly 78 per cent nitrogen gas (N2), plants cannot use this gaseous form directly -- the triple bond linking the two nitrogen atoms is extremely stable and requires specialised biological or industrial processes to break. Certain bacteria and cyanobacteria possess the enzyme nitrogenase, which allows them to fix atmospheric nitrogen into ammonia, a form plants can absorb and use. Because they work by supplementing, rather than replacing, existing soil fertility and chemical fertiliser use, and because they help restore and sustain the soil's natural microbial ecosystem rather than depleting it, biofertilisers are considered an important pillar of environmentally sustainable agriculture.

Symbiotic Nitrogen Fixation: Rhizobium

The best-known example of symbiotic nitrogen fixation is the relationship between the bacterium Rhizobium and the roots of leguminous plants (members of the pea/bean family, such as gram, pea, soybean and groundnut). Rhizobium bacteria infect the root hairs of a compatible legume host, triggering the plant to form specialised swellings on its roots called root nodules. Inside these nodules, Rhizobium fixes atmospheric nitrogen into ammonia, which the plant then uses directly, while in turn the plant supplies the bacterium with carbohydrates and a protected, oxygen-regulated niche in which to live and function -- a genuine mutualistic, two-way symbiosis. When legume crops are harvested, the nitrogen-enriched root system and nodules left behind in the soil substantially raise its fertility, which is why legumes are traditionally grown in rotation with other crops to naturally replenish soil nitrogen.

Free-Living (Asymbiotic) Nitrogen Fixers

Not all nitrogen-fixing bacteria depend on forming a close physical association with a plant root. Free-living, or asymbiotic, nitrogen-fixing bacteria live independently in the soil and fix atmospheric nitrogen without needing to colonise a host plant's tissue. Well-known examples used as biofertilisers include Azotobacter and Azospirillum, both of which are applied to the soil or seeds of non-leguminous crops (such as cereals) to boost the nitrogen available to the growing plant, even though they do not form a nodule-based partnership the way Rhizobium does with legumes.

Mycorrhiza: A Fungal Root Partnership

Mycorrhiza refers to a mutually beneficial symbiotic association between certain fungi and the roots of many plant species, including important crop plants such as Pisum (pea). In this relationship, the fungal partner -- common examples include species of the genus Glomus -- forms an extensive network of fine fungal threads (hyphae) that greatly extend the effective absorptive surface area of the plant's root system, well beyond what the plant's own roots could reach unaided. This dramatically improves the plant's ability to take up phosphorus and water from the soil, particularly in soils where phosphorus is otherwise poorly available, and some mycorrhizal associations also confer resistance to soil-borne pathogens and improved tolerance of salt and drought stress. In return, the plant supplies the fungus with carbohydrates produced through photosynthesis.

Cyanobacteria: Blue-Green Algal Biofertilisers …